Advice needed. Issues with underfloor heating and hot water system for new build

[Carrera]

Hi, i am looking for advice regarding my system. The plumber/heating engineers have been going round in circles trying different things and I am not sure they're will get to a solution.

The issues -
1. When the underfloor heating is on, the hot water cylinder does not heat up.
2. When heating just the hot water cylinder, the boiler starts cycling heavily.
3. Underfloor heating individual loops, flow rates drop as more manifolds come online.

The system -
Worcester Bosch 40cdi conventional, grundfos Magna 1 pump, Ariston 500 litre hot water cylinder, all located in a basement plant room. There are 6 underfloor heating manifolds, with ports varying from 6 to 10 with differing loop lengths, arranged over 4 floors. The system is controlled via a heatmiser network system with 30 stats in each room. There is also a towel radiator curcuit with 6 rads which we have not yet bought into the combined running equation yet.

The flow out of the boiler goes to the pump, then up to the two feeds to the manifolds ( First feed to 3 manifolds on one side of the house, ground floor, first floor and attic floor, second feed to 2 other manifolds ground floor and first floor and a third feed to the towel rad circuit). Before these feeds there is a T which feeds the hot water cylinder and the final manifold in the basement.

Temperatures at the manifolds in consistent at about 42c and the holier is outputting at 71c.

All zone valves operate correctly, all stats and timers are running and wired correctly.

Heat loss calcis have been done to size the boiler to the heat output required, indicating the boiler is oversized by about 8kw. The system has been in for about a year and it took us a while to figure out why the water was not heating properly.

Effectively when the heating is on the flow does not go through the T to the cylinder, but when the heating is off it does and therefore heats up the water.

Things that have been tried - pump has been upgraded to the one now in place, pipe work taken apart an equivalent of a low loss header using loops and spaced Ts has been tried and then removed. The T has been turned around to reduce resistance.

Any help or advice would be really appreciated as this is driving me nuts.
If I have missed vital information please let me know and I will try and answer.
many thanks in advance.

 

[Gray0689]

View attachment 16396
Don it albeit sideways!
this is the plant room setup

For the size of house all looks very small !

 

[Ermintrude]

Is that a two or 3 port on cyl? Where do the pipes at bottom on pic disappear to?
Yes the tee under the boiler behind pump also interesting? What's the total load on the heating system as it looks to be 28 and 22 mm

 

[Chalked]

The house has underfloor throughout so output may be ok.
I personally think wb heat exchangers are too restrictive.
a house of this size would have been better served with two smaller boilers on a low loss header. As underfloor does not need a lot of kw, but needs a good flow rate.

 

[Chalked]

Is that a two or 3 port on cyl? Where do the pipes at bottom on pic disappear to?
Yes the tee under the boiler behind pump also interesting? What's the total load on the heating system as it looks to be 28 and 22 mm

Zooming in its a zone valve.

 

[Gray0689]

The house has underfloor throughout so output may be ok.
I personally think wb heat exchangers are too restrictive.
a house of this size would have been better served with two smaller boilers on a low loss header. As underfloor does not need a lot of kw, but needs a good flow rate.

6500sqfoot house
6 manifolds 500litre unvented cylinder
Inch pipe all to small for my liking

 

[Ermintrude]

The house has underfloor throughout so output may be ok.
I personally think wb heat exchangers are too restrictive.
a house of this size would have been better served with two smaller boilers on a low loss header. As underfloor does not need a lot of kw, but needs a good flow rate.

Yes but if cylinder is asking for 18kw and ufh is asking for 15kw possible issue?

 

[Chalked]

At least it's working hard!!!lol

 

[Gray0689]

At least it's working hard!!!lol

That's the point it isn't working

 

[Chalked]

Quick calc for underfloor ( not exact or knowing the property)

1600 square feet = 603-87 m square.

solid floor at 21*c. Works out at roughly 37 kw! Boiler undersized.!


all this does not cure the problem with the cylinder though.

 

[Ray Stafford]

1600 square feet = 603-87 m square.

???

1600 square feet = 149 square metres.

Or am I misunderstanding you?

 

[Gray0689]

???

1600 square feet = 149 square metres.

Or am I misunderstanding you?

House is 6500 square foot ray
500litre cylinder
6 manifolds
7 towel rads

 

[Ray Stafford]

House is 6500 square foot ray
500litre cylinder
6 manifolds
7 towel rads

Gotcha. So original 1600 should have said 6500? Makes sense now. Ta

<memo to self - read the whole thread>

 

[Gray0689]

A low loss header would of been good for that set up
But to be honest I would be thinking the plumber made a hash of that as pipework is undersized
Another good job ruined by cutting corners imho

 

[Ermintrude]

A low loss header would of been good for that set up
But to be honest I would be thinking the plumber made a hash of that as pipework is undersized
Another good job ruined by cutting corners imho

Boiler is a 1/3 of the size it should be ? 604m2 @0.150kw/m2 = 90kw just for the floors! 90kw ain't ever fitting down 28mm unless it's running at 140m/s

 

[Ray Stafford]

Heat loss calcis have been done to size the boiler to the heat output required, indicating the boiler is oversized by about

Any chance of seeing those calcs?

 

[Carrera]

let me try and answer some of the questions, btw the plumber is here at the moment!
Heat loss calcs indicate heating requirement is 27 kW, done prior to boiler selection due to level of insulation, airtightness and mechanical ventilation and heat recovery systems in place.

pipes disappearing down off the picture are flow and return to the basement manifold which sits about 6 feet away

pipework is 28mm most of the way round the house to the manifolds, changing to 22 on the top floor

Flow through hot water cylinder not working even if only one manifold is running, ie extremely low heating load.

The screed on the for is the anhydrite liquid screed, which transfers heat a lot quicker than normal and is only 50 mm thick

the T referred to behind the pump? There is no t behind the pump, the one to the right is flow out, one on the left is just an elbow leading to a T to the cylinder, followed by a T to the basement manifold

 

[Chalked]

Are my eyes deceiving me? I can definitely see two pipes coming down from the boiler, both with tees?
these go off before the pump.

 

[diamondgas]

Without checking for none return valves etc I'd say your problem could be caused by the tee before the pump, unless there's a none-return valve down stream and another pump?

Already mentioned previously, by-the-way !

 

[diamondgas]

The circled bit... 🙂

 

[Carrera]

Ok I see, the two Ts one on flow and one on return leads to manual dead ends that are used to drain system, just a bit beyond the picture, and below that is the system filling loop